Abraded variable resistance control and method of manufacture

ABSTRACT

A variable resistance control is provided wherein a resistance path having an inner edge defined by a circular arc and an outer edge defined by straight lines tangent to an imaginary circular arc concentric with the circular arc defining the inner edge is supported on a nonconductive base. The straight lines intersect to define corners at the outer periphery of the resistance path. In order to bring the resistance within specified tolerances without materially affecting the linearity of the resistant path, the corners are abraded by removing a portion of the resistance material between the imaginary circular arc and the outer edge at the corners.

United States Patent Digirolamo et al.

[151 3,662,317 [451 May 9,1972

[72] Inventors: Joseph Digirolamo, Elkhart; Ronald L.

Stuckey, Berne, both of Ind.

[73] Assignee: CTS Corporation, Elkhart, Ind.

[22] Filed: Aug. 5, I970 211 App]. No.: 61,282

[56] References Cited UNITED STATES PATENTS 3,416,119 12/1968 Van Benthuysen et al. ..338/174 2,082,980 6/1937 Schellenger ..338/l95 X 2,500,605 3/1950 De Lange et al.. ...338/l 95 X 3,379,567 4/1968 Wright ..338/l95X Primary Eraminer-Laramie E. Askin Assistant ExaminerD. A. Tone Attorney-John J. Gaydos [57] ABSTRACT A variable resistance control is provided wherein a resistance path having an inner edge defined by a circular arc and an outer edge defined by straight lines tangent to an imaginary circular arc concentric with the circular arc defining the inner edge is supported on a nonconductive base. The straight lines intersect to define corners at the outer periphery of the resistance path. In order to bring the resistance within specified tolerances without materially affecting the linearity of the resistant path, the corners are abraded by removing a portion of the resistance material between the imaginary circular arc and the outer edge at the corners.

11 Claims, 5 Drawing Figures PATENTEDMM 9 I972 FIGURE- l FIGURE- 2 FIGURE- 4 FIGURE-3 INV ENTORS JOSEPH DIGIROLAMO RONALD STUCKEY BY 9% fi u m A TORNEY ROTATION FIGURE-5 wuz rwaum ABRADED VARIABLE RESISTANCE CONTROL AND METHOD OF MANUFACTURE The present invention relates to variable resistance controls and, more particularly, to an abraded resistance element for a variable resistance control and a method of making such an element.

Resistance elements are commonly made by applying a resistance material as a thin layer or film on a nonconductive support or base. Cermet resistance materials formed of a mixture of finely divided glass and a conductive fraction are deposited in a viscous mixture on a nonconductive base and fired at high temperatures to form a resistance element or film that is hard and smooth. The resistance of such a film is a function of the length, width and thickness of the deposited film. In processing these elements it is difficult to obtain the exact value desired since slight changes in the length, width and thickness of the fired film bring about variations between the desired end resistance and the actual resistance. It is important that the resistance gradients of these elements conform within exceedingly close tolerances to the specification of any circuit in which they are to be used. Accordingly it would be desirable to manufacture these elements having uniformity within specified tolerances.

In the past resistance elements have been brought within the desired tolerances by abrading a portion of the resistance film such as shown in Schellanger US. Pat. No. 2,082,980. In order to achieve predictability and fall within the desired tolerances required by the specification of the circuit, any abrupt changes in the linearity of the resistance element must be eliminated. Since the current takes the shortest possible path through a resistance element, the current density of an arcuate resistance element is highest at the inner edge. Since the current density is highest around the inner edge, it is difficult to abrade the inner edge without bringing about abrupt changes in the linearity of the resistance element. Accordingly it would be desirable to provide a method of abrading a resistance element without bringing about undesirable abrupt changes in the resistance.

Another problem associated with abrading the resistance element relates to the requirement that the surface to be traveled by the movable contactor must be smooth and uncut. In the preparation of a resistance element any deviation from the smooth uncut surface in the area wherein the movable contactor wipes the surface will result in corresponding undesirable electrical noise as the contactor moves about its path. Since the width of the film is usually quite small, the amount of correction that may be accomplished by abrading without affecting the path of the movable contactor has been limited. Accordingly it would be desirable to provide an abraded resistance element wherein the resistance path can be abraded to bring about a substantial increase in resistance without affecting the surface to be traversed by the contactor.

In the past when the inner edge has been abraded it has been found that it is preferable to merely remove a small portion around the complete inner edge thus raising the resistance without bringing about an abrupt change. With the present day desire for the miniaturization of such controls this brings about a serious limitation. Since the inner edge of the arcuate path must be spaced from the collector a distance sufficient to provide for electrical clearance, an increase of material on the inner edge to provide for abrading would result in a wider control as the spacing of the inner edge from the collector would have to be calculated for those elements which would not require abrading. Accordingly it would be desirable to provide a resistance element wherein the resistance can be increased without requiring additional material along the inside edge and corresponding increased width of the overall element.

Accordingly it is a general object of the present invention to provide a variable resistance element which can be abraded to bring the element within specified tolerances. Another object of the present invention is to provide a variable resistance ele ment having a smooth uncut path surface for traversal by a movable contactor. An additional object of the present invention is to provide a variable resistance element wherein the resistance value of the element can be varied without substantially changing the linearity of the element. Yet another object of the present invention is to provide a variable resistance element wherein the resistance value of the element can be varied without increasing the width of the element. A further object of the present invention is to provide a variable resistance element having a square pattern with abradable corners for bringing the resistance element within specified tolerances. Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelty characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Briefly, the present invention is concerned with a variable resistance element wherein a resistance material forming a resistance path is deposited on a nonconductive support. The resistance path has an inner edge partially defined by a circular arc equidistant from a center point and an outer edge lying outside an imaginary circular arc concentric with the circular arc defining the inner edge. A conductive material forming a conductive path is deposited on the nonconductive support and spaced from the inner edge of a resistance path a sufficient distance to provide for electrical clearance. Tenninals are inserted in the nonconductive support and electrically connected to the resistance path and the conductive path. In a preferred embodiment the outer edge is defined by straight lines tangent to the imaginary circular arc. The straight lines intersect to define corners at the outer periphery of the resistance path. To bring the resistance element within specified tolerances without materially affecting the linearity of the resistance element, the corners are abraded by removing a portion of the resistance material between the imaginary circular arc and the outer edge of the corner.

For a better understanding of the present invention reference may be had to the accompanying drawings wherein the same reference numerals have been applied to like parts and wherein:

FIG. 1 is an isometricview of a variable resistance control built in accord with the present invention;

FIG. 2 is a cross section of a variable resistance control embodying the present invention taken along lines II-II of FIG.

FIG. 3 is a top plan view of a variable resistance element built in accord with the present invention;

FIG. 4 is a top plan view of the variable resistance element shown in FIG. 3 with portions abraded in accord with the present invention; and

FIG. 5 is a graph showing resistance of the element before and after abrading.

Referring now to the drawings, the variable resistance control is generally indicated at 10 in FIG. 1. FIGS. 1 and 2 are substantially as shown and described in application Ser. No. 704,843 filed Feb. 12, 1968, and assigned to the assignee of the present invention. That application discloses a variable resistance control the structure of which can be used with the variable resistance element disclosed in this application and is incorporated herein by reference. As best shown in FIG. 2, the control 10 comprises a dust excluding housing 11, a rotatably supported control operating means in the form of a driver 12, a movable contactor l3, and a nonconductive base 14. Terminal pins 15 extend from the base 14 for connection to an electrical circuit.

A resistance material defining a resistance path 16 is applied to the base 14 by any suitable operation such as brushing, spraying of silk screening. The resistance material can be formed of a cermet resistance material comprising a nonconductive glass binder material having minute particles of a conductive fraction dispersed throughout such as disclosed in Faber, Sr. et al. US. Pat. No. 3,304,199 assigned to the same assignee as the present invention. After the resistance material is deposited onto the base 14 it is cured to bond the resistance material to the surface of the base 14 such as by firing at a suitable temperature to fuse the glass particles into a glass matrix as well known in the art. As shown in FIG. 3 the resistance path 16 has an inner edge partially defined by a circular arc l7 and an outer edge defined by intersecting straight lines 18. The straight lines 18 are tangent to an imaginary circular arc 19 that is concentric with the circular are 17 defining the inner edge of the resistance path 16. Preferably, the outer edge of the resistance path 16, as defined by the straight lines 18, is spaced from the outer edge of the base 14. This is especially desirable where a metal housing is utilized so as to provide electrical clearance between the outer edge of the resistance path 16 and the metal housing. Intersection of the straight lines 18 forms corners and defines an overall U- shaped resistance path with a bottom 20 and a pair of legs 21. Conductive material in the form of conductive pads 22 is deposited on the base 14 at the end of the legs 21 to electrically connect the resistance path 16 to terminals 15a and 150 through a solder 31 surrounding the terminals 15a, 15:. Conductive material in the form of a center collector 23 is deposited on the base 14 and electrically connected to terminal 15b by means of deposited conductive material forming a conductive path 24 and solder 31 surrounding the terminal 15b.

It is obvious that the configuration of the resistance path 16 as shown in FIG. 3 provides for the optimum amount of resistance material that can be deposited on the base 14 and maintain the desired electrical clearance between the inner edge and the collector and between the outer edge and the housing. Previously a resistance path would have an inner edge corresponding to the circular are 17 shown in FIG. 3 and an outer edge corresponding to the imaginary circular arc 19. When using mechanized assembly and printing procedures it is desirable to have a square base for ease of handling. Accordingly the outer edge of the base would have to be spaced a sufficient distance from the imaginary circular are 19 to provide the desired electrical clearances. Since the straight lines 18 are tangent to the imaginary circular are 19 and parallel to the outer edge of the base 14, the addition of the material outside of the imaginary circular are 19 does not result in the need to widen the base 14. When it was desired to abrade the prior art resistance elements around the inside edge of the resistance path, a corresponding increase in the width of the base was required to provide the necessary electrical clearances. The collector 23 must be spaced from the circular are 17 a distance sufficient to provide for electrical clearance, such distance being dependent upon the particular parameters of the circuit for which the resistor is to be used. If the resistance path 16 were printed with additional material along the inside edge the distance between the inner edge as defined by circular are 17 and the collector 23 must be determined for the element as originally printed since some and hopefully many of the elements will fall within the desired tolerances and not require abrading. Therefore if the width of the resistance path between the circular are 17 and the imaginary circular are 19 were untouched so as to provide for a smooth and uncut surface for the contactor 13 to travel, then the outer edge defined by the imaginary circular are 19 as shown in FIG. 3 would have to be spaced outwardly from the present position shown in FIG. 3. Accordingly the width of the base 14 would have to be enlarged to provide for the necessary electrical clearance between the housing and the outer edge of the resistance path 16.

Ordinarily the resistance change for each incremental movement of an electrical contactor in contact with the resistance path is constant where the resistance path is formed of a homogenous resistance material and the thickness and width of the path are uniform. This constant incremental change produces a linear resistance element. Accordingly one would not expect a linear resistance element from the path shown in FIG. 3 since the width of the path varies from one point to another. One would expect a large departure in linearity at the comers of the resistance path 16 due to the large amount of resistance material lying between the straight lines 18 and the imaginary circular are 19. Surprisingly, this is not the case. In FIG. 5, the curve 25 corresponds to the resistance versus rotation curve of a resistance element such as shown in FIG. 3. While there are two slight depressions in the curve 25 corresponding to the corners of the resistance path 16, the curve 25 does not have any abrupt changes in the resistance value and will conform within specified tolerances for the majority of applications. Thus it would not be necessary to abrade those elements that met the desired end resistance since the curve 25 is substantially linear and without undesirable abrupt changes. The substantial linearity of the resistance path 16 shown in FIG. 3 can be best explained by the fact that there is a low current density at the corners and accordingly the additional resistance material in the corners is thought to have little effect on the overall resistance value of the element. This is because more electrical current will flow along the inside edge defined by circular are 17 resulting in a higher current density at the inside edge and a lower current density at the outside edge.

Referring now to FIG. 4 the resistance path 16 is shown having its upper right and left-hand comers abraded in two separate embodiments. In the upper left-hand corner a denuded area 26 separates the resistance path 16 from a comer portion 27. It is not necessary to have a very large denuded area 26 since the current will flow along the resistance path 16 which is of much lower resistance than the infinite resistance across the denuded portion 26. The upper right-hand comer of the resistance path 16 has been abraded leaving the denuded area 28. The resistance material may be abraded by any suitable method, the choice of which will determine the ultimate configuration. The denuded areas 26 and 28 are bounded on the side of the resistance path 16 by a straight line tangent to the imaginary circular arc 19. The area could extend to the imaginary circular arc 19, however, it is obviously much simpler to abrade a straight portion than to attempt to follow the circular contour of the imaginary circular are 19. This is another advantage of the instant application since the care and precision required in abrading around the inside edge are eliminated by the much simpler method of abrading a straight path through the resistance material at the corner of the resistance path 16. In view of the fact that the U- shaped resistance path as shown in FIG. 3 results in a substantially linear curve as shown by curve 25 in FIG. 5, one would make two presumptions relating to abrading the comers of such a pattern. One being that if the corners were abraded they would not result in a substantial increase in resistance; the other being that if there were an increase there would be abrupt changes due to the substantial reduction in the amount of material at those portions. Surprisingly, neither of these presumptions are accurate. Curve 29, shown in FIG. 5, dis closes the resistance versus rotation curve of a resistance element after abrading the corner portions. Curve 29 clearly results in a substantial increase in the end resistance over the unabraded curve 25 and also results in a more linear curve. It is easy to explain why curve 29 is more linear than curve 25 since by abrading the corners we have removed the portions of the resistance path 16 that resulted in curve 25 having the slight depressions. It is not fully understood why the substantial increase is achieved over an unabraded resistance element in view of the unabraded elements linearity being explained by the low current density at the corners, however, tests have shown that as much as a 35 percent increase can be obtained without affecting linearity appreciably. For an even greater percentage increase in the overall resistances, the portions 30 lying outside of the imaginary circular are 19 can be removed.

From the above description it will be apparent that a very simple method has been provided for abrading a variable resistance element to bring the same into tolerance. While there has been illustrated what is at present considered to be a preferred embodiment of the present invention and modification thereof it will be appreciated that numerous changes or modifications are likely to occur to those skilled in the art and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. The method of making a resistance element for a variable resistance control comprising the steps of:

a. depositing a resistance material on a nonconductive base,

said material defining a resistance path being generally U- shaped with a bottom and a pair of legs, an inner edge of said resistance path being partially defined by a circular arc and an outer edge of said resistance path being defined by straight lines tangent to an imaginary circular arc with said circular are partially defining said inner edge, said straight lines being parallel to edges of said base,

b. providing a center collector on the nonconductive base, and center collector being spaced from the inner edge of said path to provide electrical clearance between said path and said collector,

c. providing terminals in electrical connection with said resistance path, and

d. removing a portion of the resistance path lying between said imaginary circular arc and said outer edge.

2. The method of making a resistance element for a variable resistance control comprising the steps of:

a. depositing a resistance material on a surface of a nonconductive base, said material defining a resistance path being generally U-shaped with a bottom and a pair of legs, an inner edge of said resistance path being partially defined by a circular arc and an outer edge of said resistance path being partially defined by intersecting straight lines forming at least one comer at the bottom of the U-shaped path,

b. depositing conductive material in the form of a center collector on said surface of the nonconductive base, said center collector being spaced from the inner edge of said path to provide electrical clearance between said path and said collector,

c. inserting terminals in said base in electrical connection with said resistance path and said collector, and

d. abrading a portion of the outer edge of the resistance path at said corner.

3. The method of making a resistance element for a variable resistance control comprising the steps of:

a. depositing a resistance material on a nonconductive base,

said material defining a resistance path being generally U- shaped with a bottom and a pair of legs, an inner edge of said path being partially defined by a circular arc and an outer edge of said resistance path being partially defined by straight lines tangent to an imaginary circular are concentric to the circular arc defining said inner edge, said straight lines intersecting to form at least one corner at the bottom of the resistance path,

b. depositing conductive material in the form of a center collector on the nonconductive base, said center collector being spaced from the inner edge of said path to provide electrical clearance between said path and said collector,

c. inserting terminals in said base in electrical connection with said resistance path and said collector, and

d. abrading a portion of the outer edge of the resistance path at said corner.

4. A variable resistance control comprising a base, a plurality of terminals extending from said base for connection to an electrical circuit, a driver supported for rotation relative to the base, a resistance path supported on a surface of said base, said resistance path being generally U-shaped with a bottom and two legs, an inner edge of said path being partially defined by a circular arc and an outer edge of said path being partially defined by straight lines, said outer edge having contiguous denuded areas at the bottom of the generally U-shaped path, a

collector S l.l!ppOfled on said surface and spaced from the inner edge of san path, one of said straight hnes being tangent to an imaginary circular arc concentric to the circular are partially defining said inner edge at the bottom of said path, two of said straight lines spaced on opposite sides of said collector being perpendicular to said one and tangent to said imaginary circular arc, conductive material deposited on said base electrically connecting said resistance path and said collector with said terminals, and a contactor constrained to rotate upon rotation of said driver for wipingly engaging said collector and said resistance path.

5. The variable resistance control of claim 4, wherein said contiguous denuded areas have a boundary tangent to said imaginary circular are.

6. A variable resistance control comprising a base, a driver supported for rotation relative to the base, a resistance path supported on a surface of said base, said resistance path being generally U-shaped with a bottom and two legs, an inner edge of said path being partially defined by a circular arc, an outer edge of said path being partially defined by straight lines and having contiguous denuded areas at the bottom of the generally U-shaped path, a collector supported on said surface and spaced from the inner edge of said path, one of said straight lines being tangent to an imaginary circular are concentric to the circular are partially defining said inner edge at the bottom of said path, two of said straight lines spaced on opposite sides of said collector being perpendicular to said one and tangent to said imaginary circular are, said one and said two being parallel to edges of said base, terminals electrically connected to said resistance path for connection to an electrical circuit, and a contactor constrained to rotate upon rotation of said driver for wipingly engaging said collector and said resistance path.

7. The variable resistance control of claim 6, wherein said contiguous denuded areas have a boundary tangent to said imaginary circular are.

8. A method of making a resistance element for an electrical control comprising the steps of:

a. depositing a thin film of resistance material on a surface of a dielectric base, said thin film being defined by a' generally U-shaped configuration with a bottom and a pair of legs, the inner edge of said thin film being partially defined by a circular arc and an outer edge of said thin film being defined by straight lines tangent to an imaginary circular arc concentric with said circular are partially defining said inner edge, said straight lines being parallel to edges of said base,

b. curing the thin film deposited on the surface of the dielectric base and produce a resistance path,

c. connecting terminals to the legs of the resistance path,

d. measuring the resistance between the pair of terminals,

and

e. removing a portion of the resistance path lying between the imaginary circular arc and the outer edge of the dielectric base to increase the resistance of the resistance path.

9. The method of claim 8, wherein the step of depositing the thin film of resistance material comprises the step of screening the thin film of resistance material onto the surface of the dielectric base.

10. The method of claim 8, wherein portions of the outer edge of the thin film at the junctions between the bottom and each of the legs are defined by a pair of square corners and the step of removing comprises the removing of a narrow strip of resistance path diagonally of the corners to increase the resistance value of the resistance path connected to the terminals.

11. The method of claim 8, wherein the resistance material is cermet and the step of curing comprises firing the thin film deposited on the base at a temperature sufficient to bond the thin film of cermet to the surface of the base. 

1. The method of making a resistance element for a variable resistance control comprising the steps of: a. depositing a resistance material on a nonconductive base, said material defining a resistance path being generally Ushaped with a bottom and a pair of legs, an inner edge of said resistance path being partially defined by a circular arc and an outer edge of said resistance path being defined by straight lines tangent to an imaginary circular arc with said circular arc partially defining said inner edge, said straight lines being parallel to edges of said base, b. providing a center collector on the nonconductive base, and center collector being spaced from the inner edge of said path to provide electrical clearance between said path and said collector, c. providing terminals in electrical connection with said resistance path, and d. removing a portion of the resistance path lying between said imaginary circular arc and said outer edge.
 2. The method of making a resistance element for a variable resistance control comprising the steps of: a. depositing a resistance material on a surface of a nonconductive base, said material defining a resistance path being generally U-shaped with a bottom and a pair of legs, an inner edge of said resistance path being paRtially defined by a circular arc and an outer edge of said resistance path being partially defined by intersecting straight lines forming at least one corner at the bottom of the U-shaped path, b. depositing conductive material in the form of a center collector on said surface of the nonconductive base, said center collector being spaced from the inner edge of said path to provide electrical clearance between said path and said collector, c. inserting terminals in said base in electrical connection with said resistance path and said collector, and d. abrading a portion of the outer edge of the resistance path at said corner.
 3. The method of making a resistance element for a variable resistance control comprising the steps of: a. depositing a resistance material on a nonconductive base, said material defining a resistance path being generally U-shaped with a bottom and a pair of legs, an inner edge of said path being partially defined by a circular arc and an outer edge of said resistance path being partially defined by straight lines tangent to an imaginary circular arc concentric to the circular arc defining said inner edge, said straight lines intersecting to form at least one corner at the bottom of the resistance path, b. depositing conductive material in the form of a center collector on the nonconductive base, said center collector being spaced from the inner edge of said path to provide electrical clearance between said path and said collector, c. inserting terminals in said base in electrical connection with said resistance path and said collector, and d. abrading a portion of the outer edge of the resistance path at said corner.
 4. A variable resistance control comprising a base, a plurality of terminals extending from said base for connection to an electrical circuit, a driver supported for rotation relative to the base, a resistance path supported on a surface of said base, said resistance path being generally U-shaped with a bottom and two legs, an inner edge of said path being partially defined by a circular arc and an outer edge of said path being partially defined by straight lines, said outer edge having contiguous denuded areas at the bottom of the generally U-shaped path, a collector supported on said surface and spaced from the inner edge of said path, one of said straight lines being tangent to an imaginary circular arc concentric to the circular arc partially defining said inner edge at the bottom of said path, two of said straight lines spaced on opposite sides of said collector being perpendicular to said one and tangent to said imaginary circular arc, conductive material deposited on said base electrically connecting said resistance path and said collector with said terminals, and a contactor constrained to rotate upon rotation of said driver for wipingly engaging said collector and said resistance path.
 5. The variable resistance control of claim 4, wherein said contiguous denuded areas have a boundary tangent to said imaginary circular arc.
 6. A variable resistance control comprising a base, a driver supported for rotation relative to the base, a resistance path supported on a surface of said base, said resistance path being generally U-shaped with a bottom and two legs, an inner edge of said path being partially defined by a circular arc, an outer edge of said path being partially defined by straight lines and having contiguous denuded areas at the bottom of the generally U-shaped path, a collector supported on said surface and spaced from the inner edge of said path, one of said straight lines being tangent to an imaginary circular arc concentric to the circular arc partially defining said inner edge at the bottom of said path, two of said straight lines spaced on opposite sides of said collector being perpendicular to said one and tangent to said imaginary circular arc, said one and said two being parallel to edges of said base, terminals electrically connected to said resistance path for connEction to an electrical circuit, and a contactor constrained to rotate upon rotation of said driver for wipingly engaging said collector and said resistance path.
 7. The variable resistance control of claim 6, wherein said contiguous denuded areas have a boundary tangent to said imaginary circular arc.
 8. A method of making a resistance element for an electrical control comprising the steps of: a. depositing a thin film of resistance material on a surface of a dielectric base, said thin film being defined by a generally U-shaped configuration with a bottom and a pair of legs, the inner edge of said thin film being partially defined by a circular arc and an outer edge of said thin film being defined by straight lines tangent to an imaginary circular arc concentric with said circular arc partially defining said inner edge, said straight lines being parallel to edges of said base, b. curing the thin film deposited on the surface of the dielectric base and produce a resistance path, c. connecting terminals to the legs of the resistance path, d. measuring the resistance between the pair of terminals, and e. removing a portion of the resistance path lying between the imaginary circular arc and the outer edge of the dielectric base to increase the resistance of the resistance path.
 9. The method of claim 8, wherein the step of depositing the thin film of resistance material comprises the step of screening the thin film of resistance material onto the surface of the dielectric base.
 10. The method of claim 8, wherein portions of the outer edge of the thin film at the junctions between the bottom and each of the legs are defined by a pair of square corners and the step of removing comprises the removing of a narrow strip of resistance path diagonally of the corners to increase the resistance value of the resistance path connected to the terminals.
 11. The method of claim 8, wherein the resistance material is cermet and the step of curing comprises firing the thin film deposited on the base at a temperature sufficient to bond the thin film of cermet to the surface of the base. 